1. Field of the Invention
The present invention relates to a method of and an apparatus for generating a proof image for a printed image before the printed image is produced from an original image by a printed image generating apparatus.
2. Description of the Related Art
In the art of printing and plate making, there have widely been used printed image generating apparatus, or image setters, for electrically processing image information of originals to generate film plates for printing images for the purposes of simplifying the processing steps and improving the image quality. The image formed on a film plate which is generated by an image setter is transferred to a printing plate, which will be used to produce a printed material.
In recent years, there have become available a CTP (Computer To Plate) apparatus capable of directly producing a printing plate from image information without producing a film plate and a CTC (Computer To Cylinder) apparatus capable of producing an image on the plate cylinder of a printing press from image information.
These printed image generating apparatus operate by reading an original image with a scanner to generate continuous tone image data, processing the continuous tone image data as desired, comparing the processed continuous tone image data with threshold data to generate binary image data, and controlling a laser beam or the like according to the binary image data to form an image on a recording medium for printing.
When the original image is converted into a halftone dot image, a periodic nature of the original image interferes with the periodic nature of halftone dots, producing a moire pattern in the outputted printed image and an image structure peculiar to halftone dots, such as a rosette image.
In order to confirm such a moire pattern and an image structure in advance, predict the characteristics of a printed image, and set optimum printing conditions, there has heretofore been used a proof image generating apparatus of simple arrangement for generating a proof image which simulates a printed image.
The proof image generating apparatus sets the same output conditions as the output resolution and screen ruling which have been set by the printed image generating apparatus, and generates a proof image under the set output conditions for predicting and inspecting a printed image. In view of the trends of the printed image generating apparatus toward higher output resolutions, the proof image generating apparatus has output resolutions and screen rulings equivalent to those of the printed image generating apparatus so as to be able to output proof images at the output resolutions of the printed image generating apparatus.
Recently, a technique has been developed which is capable of setting output conditions of a low output resolution and a high screen ruling and outputting a highly fine printed image of high gradations. Specifically, there has been developed a super-cell printed image generating apparatus which provides a small number of gradations that can be expressed by individual halftone dots because the output resolution is low, but can produce a high-gradation image by combining a plurality of halftone dots. The super-cell printed image generating apparatus is capable of outputting an image at an output resolution of 1200 dpi (dots per inch) and a screen ruling of 175 lpi (lines per inch), which has heretofore been outputted at an output resolution of 2400 dpi and a screen ruling of 175 lip. Therefore, the super-cell printed image generating apparatus can output printed images at a high speed.
However, the output conditions of the proof image generating apparatus remain equivalent to those of the printed image generating apparatus. In order to change the output conditions, it is necessary to substantially modify the arrangement of the proof image generating apparatus, and such substantial modifications entail a considerable expenditure of expenses.